Field of the Invention
The invention relates to a process and apparatus for converting nondistillable hydrocarbons, such as asphaltenes or tar sands into lighter products in nozzle upstream of or within an FCC riser reactor.
Processes for the cracking of hydrocarbon feedstocks via contact at appropriate temperatures and pressures with fluidized catalytic particles are known in the art generically as "fluid catalytic cracking" (FCC).
Relatively low boiling point hydrocarbons, such as gas oils, are preferred feedstocks for FCC operations. Such hydrocarbons generally contain fewer contaminants and have a lower tendency to produce coke during the cracking operation than heavier hydrocarbons. However, the relatively low content of such light hydrocarbons in many current crude mixes has driven refiners to use heavier feeds, e.g., residual oils, as feedstocks to the FCC operation. Heavier feeds generally contain more metals which contaminate the catalyst and large complex hydrocarbons structures such as asphaltenes and porphyrins. These large, non-distillable hydrocarbons, sometimes referred to as Conradson Carbon Residue (CCR), or asphaltenics, usually are converted into coke, rather than cracked to lighter products. This increases the yield of coke during the cracking operation.
The high viscosity of these heavy fractions also makes them difficult to vaporize. Conventional approaches at feed atomization create large droplets of resid, which may not vaporize until half way up the riser, or even later, so the heavy ends are converted thermally. Some heavy liquid may be swept with spent catalyst into the regenerator and burned, though it was potentially convertible.
The difficulty of converting resids into lighter products depresses the value of this portion of the crude and provides great incentive for cracking it in FCC. Its use as FCC feed causes problems: metals contamination, feed atomization and feed vaporization. While use of metals scavengers, or catalysts which can tolerate large metals levels, or more frequent catalyst replacement, provides refiners with a way to deal with the metals problems, the resids are still hard to atomize and vaporize.
The state of the art on dealing with resid feeds will be reviewed, and then FCC feed atomization will be reviewed.